This invention relates to a mobile telephone terminal and a method of preventing the unlawful use thereof. More particularly, the invention relates to a mobile telephone terminal, as well as a method of preventing its unlawful use, in which it is possible to prevent communication achieved by unlawful use of the terminal by illegally writing a first ID code for apparatus identification and a second ID code such as a telephone number in a non-volatile memory of the mobile telephone terminal.
A mobile telephone terminal (portable telephone terminal or car telephone terminal) used in an AMPS (advanced mobile phone system) of the kind employed in North America, Australia and some other countries is provided with two types of ID numbers referred to as a MIN (mobile identification number ) and an ESN (electronic serial number). These ID numbers are stored in a non-volatile memory such as an EEPROM (electrically erasable programmable read-only memory) incorporated within the terminal. MIN corresponds to the telephone number and ESN corresponds to the manufacturing serial number. When a call is placed, these two ID codes are transmitted from the mobile telephone terminal to the network, and whether the two ID codes have been registered or not is checked on the network side. A speech path is established between the calling and called parties if it found that the two ID codes have been registered, and the calling party is billed in accordance with the duration of the call and the distance. However, there are instances in which the MIN and ESN of the legal user are acquired unlawfully and written in the EEPROM of another mobile telephone terminal, which is then used for unlawful communication. Communication by such unlawful use of a mobile telephone terminal is almost impossible to deal with on the exchange side, as a result of which the legitimate user is billed for use of the line. Consequently, there is demand for a method of preventing unlawful use of mobile telephone terminals, through which method it is possible to refuse communication based upon unlawful use of the mobile telephone terminal.
The AMPS system is a mobile phone system of the cellular type. As shown in FIG. 11, a service region is subdivided into a number of cells (wireless zones) 1.sub.1, 1.sub.2, 1.sub.3, . . . , and the cells are provided with individual wireless base stations 2.sub.1, 2.sub.2, 2.sub.3, . . . , respectively. The wireless base stations are formed into groups each having several of the base stations, a wireless line-control station (not shown) is provided for each such group of wireless base stations, the wireless line-control stations are connected to a mobile phone exchange (hereinafter referred to as a car phone exchange) 3, and the car phone exchange 3 is connected to an exchange 4 of a public telephone network.
The cellular-type mobile phone system is so designed that as a portable telephone terminal 5 or car telephone terminal 6 moves, the wireless base stations that send and receive the radio waves are changed over from one to another so that communication can be achieved with an indoor fixed telephone 7 or another mobile telephone terminal. This makes it possible to carry on telephone conversations over a wide area and to increase the number of channels. The number of lines can be increased without limit as well.
As mentioned above, the two ID numbers (MIN and ESN) are stored in an internal non-volatile memory, such as an EEPROM, of the AMPS-type mobile telephone terminal, the two ID codes are sent to a network from the mobile telephone terminal when a call is placed, a check is made on the network side to determine if the two ID codes have been registered, and a speech path is established between the calling and called parties only if the two ID codes have been registered.
The ESN corresponds to the manufacturing serial number and is written in the EEPROM of the mobile telephone terminal by the manufacturer of the terminal. The mobile telephone terminal is then shipped. The MIN corresponds to the telephone number and is written in the EEPROM by the provider of communication service. More specifically, the user who has purchased the mobile telephone terminal brings the terminal to, say, the dealer of a communication service and registers the terminal so that it may receive the communication service. The dealer provides the mobile telephone terminal with the MIN and writes the MIN in the EEPROM of the terminal. The provider of the communication service reads the ESN, which has been written in the EEPROM of the mobile telephone terminal by the manufacturer, and registers the set of the ESN and MIN with the network.
When the two ID codes constituted by the ESN and MIN have thus been written in the EEPROM of the mobile telephone terminal and registered with the network, a call is capable of being placed using the mobile telephone terminal.
When power is supplied to the mobile telephone terminal, the terminal scans the paging channels of the surrounding wireless ground stations, receives the paging waves transmitted at all times from each wireless ground station and tunes to the channel of the station having the strongest signal. Thereafter, the terminal receives position information over this channel and compares this with position information already stored. If the two items of position information differ, the terminal reads the two ID codes ESN and MIN out of the EEPROM, transmits them to the wireless ground station over the paging channel and registers position with the car phone exchange 3. As a result, the mobile telephone terminal enters a waiting mode. When the mobile telephone terminal moves and the strength of the received signal weakens, the terminal performs scanning again.
When an outgoing call is placed, the calling party enters the other party's telephone number from a control panel and presses a transmission key, whereupon the mobile telephone terminal reads the two ID codes ESN and MIN out of the EEPROM. These ID codes and the called party's telephone number are then transmitted to the wireless ground station over the paging channel, and the wireless ground station responds by transmitting the received information to the wireless line-control station (not shown). The latter decides the speech channel, notifies the mobile telephone terminal via the wireless ground station and informs the car phone exchange 3 of the outgoing-call information and speech channel that has been decided. The car phone exchange 3 checks to determine whether the set of two ID codes ESN, MIN received has been registered and, if they have, connects the speech channel to the public telephone system. The mobile telephone terminal can then communicate with the called party's terminal via the designated speech channel. Further, the car phone exchange 3 increments the message rate, which is stored in correlation with the MIN of the calling terminal, in conformity with the duration of the call and call distance and bills the calling party accordingly. If the set of two ID codes ESN, MIN received has not been registered, the car phone exchange 3 does not connect the speech channel to the public telephone network.
Accordingly, if the MIN and ESN are registered with the network and written in the EEPROM of the mobile telephone terminal, then the terminal can be utilized. This means that an illegal user is capable of illicitly acquiring the MIN and ESN of the mobile telephone terminal possessed by the rightful user, writing the MIN and ESN in the EEPROM of another mobile telephone terminal and using the other terminal for communication. This can subject the rightful user, the provider of the communication service and the manufacturer of the mobile telephone terminal to tremendous losses. The technique employed by such an individual to unlawfully acquire the MIN and ESN of the mobile telephone terminal is to read out the data stored in the EEPROM of the terminal and analyze this data, whereby the MIN and ESN can be determined.
A conventional measure taken on the terminal side in order to prevent unlawful use is to encrypt the ID codes on the terminal side before writing them in the EEPROM, with the ID codes being deciphered and then transmitted at the time of communication. With such an expedient, the ID codes cannot be analyzed from the EEPROM by hardware means and therefore cannot be acquired in a simple manner.
Even if the measure described above is taken, however, ID codes such as the MIN and ESN can still be acquired by intercepting the communication between the mobile telephone terminal and the wireless ground station. In actuality, this method is already being used to acquire the ID codes unlawfully, encrypt and write them in the EEPROM of another mobile telephone terminal and use the other mobile telephone terminal for communication. Thus, the conventional method of making it impossible to read the ID codes out of the EEPROM is not satisfactory for preventing unlawful use of mobile telephone terminals.